Seventeen tautomeric structures of hemihexaphyrazine were considered using DFT in B3LYP/pcseg-2 approximation. The tautomer of D3h symmetry was found to be the most energetically favorable and transformations into other tautomers are practically forbidden by high energy barriers (more than 50 kJ mol-1). The energetics of hydrogen bonding was estimated by NBO and QTAIM calculations and the preferable D3h tautomer turned out to possess the lowest total hydrogen bond stabilization energy, according to the results of the NBO-analysis. Aromaticity of the D3h and C3h tautomers of hemihexaphyrazine and its thiadiazole annulated analogue was described with use of three popular descriptors: NICS, HOMA and FLU. Tautomeric preference and structural features of the both molecules were found to be very similar.